The present invention pertains generally to nonfocusable optics (i.e., nonrefractive and nonreflective) and more particularly to coded aperture imaging. Other coded aperture imaging devices are disclosed in U.S. Pat. No. 4,209,780, entitled "Coded Aperture Imaging With Uniformly Redundant Arrays," by Edward E. Fenimore et al., issued June 24, 1980. This invention is a result of a contract with the Department of Energy (Contract W-7405-ENG-36).
Uniformly redundant arrays used for coded aperture imaging were first disclosed by Edward E. Fenimore et al. in the above-referenced U.S. patent, Applied Optics, 17, p. 337, Feb. 1, 1980, Applied Optics, 17, p. 3562, Nov. 16, 1978, Applied Optics, 18, p. 945, Apr. 1, 1979, and in Los Alamos Scientific Laboratory Mini-Review, LASL-78-102, January 1979. These references clearly point out the superior advantages of the use of uniformly redundant arrays in coded aperture imaging. For example, uniformly redundant arrays can be used to image X-ray radiation with very low noise characteristics. X-rays have many uses, from every day medical diagnostics, to probing the mysterious cosmic black holes. In many experiments, X-rays provide new and often unique information. The ability of X-rays to characterize the temperature and chemical make-up of material has been exploited in such fields as fusion energy and solar physics to gain insight into the physical processes that will some day help provide our energy needs.
A major difficulty in using information carried by X-rays is the difficulty in imaging X-ray information. In contrast to ordinary light, X-rays cannot be reflected readily by mirrors (nonreflective) or bent by lenses (non-refractive). Because ordinary cameras or telescopes are ineffective in forming an X-ray image, it is difficult to determine the location of the X-ray emitting source or structure within the source.
The single pinhole camera has been successfully used to image X-ray information. The single pinhole camera consists of a small hole in an otherwise opaque material, with a piece of film mounted at a predetermined distance behind the pinhole. An image is formed because the small hole restricts the viewing angle from any particular point on the film to only one small part of the emitting source. However, to provide resolution with a single pinhole camera the hole must be made as small as possible. If the hole is large, the X-ray intensity from the large area of the source will be averaged together, making it impossible to distinguish (resolve) smaller features. On the other hand, a small hole often has insufficient area to collect enough X-rays to produce an interpretable picture. In other words, the image produced by the pinhole camera has insufficient intensity. The conflict between the need for small hole to obtain resolution and the need for a large hole to obtain sufficient intensity often limits the usefulness of the X-ray pictures obtainable from a single pinhole camera.
Coded aperture imaging using uniformly redundant arrays, as set forth in the above-disclosed references, overcomes the conflicting requirements for imaging with a single pinhole camera. Images produced in this manner have flat side lobes resulting in the elimination of ghosts and extraneous noise common in conventional coded aperture imaging systems.
X-rays in the 1-10 keV energy range are of particular interest in many fields such as laser fusion and X-ray astronomy. Use of uniformly redundant array patterns for imaging in the 1-10 keV energy range is of great interest in many technical arts. However, uniformly redundant arrays, such as set forth in the above-disclosed references, are not a self supporting structure. Consequently, a supporting substrate must be used to support the uniformly redundant array apertures disclosed above. Unfortunately, however, throughput is greatly reduced by the use of substrates in low-energy applications such as 1-10 keV X-rays. Reduction of throughput conflicts directly with the intended purpose for using the uniformly redundant array coded aperture imaging system.